Actinic radiation is a type of radiation which may induce chemical reactions. The ultraviolet portion of sunlight, for example, is an important actinic radiation. As the effect of actinic radiation on living organisms may be very harmful, all parts of the body which may be exposed to actinic radiation must be protected, and this is particularly true of human skin. In order to protect the skin against actinic radiation, it is common practice to apply preparations to the skin having agents which tend to essentially absorb the actinic radiation.
One well known compound for this purpose is, for example, p-aminobenzoic acid, but this compound has the serious drawback of showing insufficient absorption in the ultraviolet portion of the spectrum.
More recently purines and pyrimidines, as well as purine derivatives, such as the fluoro, chloro and methyl derivatives of purines, have been recommended as ultraviolet absorbing agents. However, these compounds do not show the degree of solubility which are required for the practical use of these compounds in protective preparations. Moreoever, they show only little absorption between 290 and 320 nm.
Even more important is the following phenomenon. It is known from radiation biology that ultraviolet radiation has a harmful effect on DNS and RNS. This results in the dimerization of two adjacent pyrimidine bases. This dimerization cannot be reversed by, for example, heat treatment. The most frequent dimer is the thymine dimer which may occur in the form of 6 possible isomers. The biological significance of the thymine dimers may be recognized by the fact that with decreasing survival rate of transformability of bacteria the number of dimers increases. Most recently, such ultraviolet damage has also been observed on human epidermic cells. The formation rate of dimers is at a maximum at 280 nm, whereas ultraviolet radiation at about 240 nm is capable of cleaving dimers which have already been formed. This is in agreement with the biological observation that the damage in bacterial DNS caused by radiation at 280nm may be reversed by a second radiation at 240 nm. Although the thymine dimers represent the most important and most frequent lethal damage caused by ultraviolet radiation, dimers of the other pyrimidines and other types of defects may play also an important role.
From the foregoing, it is evident that a protective agent against actinic radiation in the ultraviolet area should not only protect against the erythemae generating wave lengths between 290 nm and 315 nm, with an erythemae maximum at 300 nm, but should also shield against ultraviolet radiation at about 280 nm, while, on the other hanad, it should show the highest possible radiation transmittance at 240 nm. This would have the effect that erythemae and damage at the pyrimidine bases of the nycleic acids are prevented and at the same time the pyrimidine dimers which may already be formed are reversed.